We begin with a mystery and a manhunt unfolding right now. Hundreds of fbi agents, texas rangers, u.S. Marshalls are joining forces trying to track down the person or the gang targeting law enforcement and their families. Two weeks ago, a case in colorado. Now in texas a second case. And a law enforcement official being gunned down. The question, are these cases connected? Abc's senior justice correspondent pierre thomas on the race to make sure this doesn't happen again. Today throughout texas, fear and signs of tighter security. The kauffman county courthouse, employees being walked in by armed guard. No prosecutions here today. The district attorney's office is closed after the city's top prosecutor and his wife were murdered in cold blood saturday. They were shot multiple times with an assault rifle after an unknown gunman broke into their home. I would be less than honest if I told you that I was not worried. Kauffman district attorney mike McLelland's murder came months after one of his deputies, mark hasse, was executed by two masked gunman. At the time he vowed his killer would be caught. We're going to find you. Reporter: Now mclelland has been murdered and prosecutors are on alert. We're under ck. Reporter: One prosecur talked about a w from his wife as he left for work. She told me to be careful and told me that she loved me and i told her to be careful as well. We'll be calling one another quite a bit during the day. Will. Reporter: Police are going through the prosecutor's cases trying to figure out who would want them dead. There are prime candidates. This is, I think, a clear concern to iividuals who are in public life, particularly those who deal with some very mean and vicious individuals, whether they're white supremacy groups or the drug cartels that we have. Reporter: The kauffman district attorney's office recently prosecuted members of a prison gang called aryan brotherhood of texas. Prosecutors want to know if they were out for revenge. Pierre is joining us now. This is extremely rare in the united states, the targeting of law enforcement. So where does this lead? What does the trail say about who might be behind it? Diane, it really is. This is only the 13th time in the united states since 1960 that a prosecutor has been killed. It's much more common in mexico where the cartels are king. Known for this kind of violence. So the white supremacist angle is one of a number being pursued.

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